Experimental and numerical study on the flexural strengthening of rolled steel beams using intermediate modulus pultruded Carbon Fibre Reinforced Polymer (CFRP) composite sheets is presented in this paper. De-bonding is the commonly observed failure mode in external strengthening of steel structural elements using CFRP composite sheets. One control and seven strengthened steel beams bonded with CFRP composite sheets along with different anchorage systems such as mechanical fasteners and intermittent carbon fibre fabric composite wraps were tested up to failure. The influence of anchorage systems on load/deflection behaviour, load-carrying capacity, toughness, stiffness, failure strain, failure mode, and deflection ductility of strengthened beams were found from the test data. Finite element (FE) models of the specimens generated using FEA software ABAQUS were analysed and compared with experimental data. The test results show that steel beams with external bonding along with intermittent U-wraps have better performance in terms of load-carrying capacity, toughness, and deflection ductility. The increase in load carrying capacity was up to 19% and the maximum strain in CFRP sheet reached up to 69% of its rupture strain. Parametric studies were also conducted for strengthening steel girders to find the effect of dimensions and orientation of CFRP composites used in the strengthening schemes. © 2021 Institution of Structural Engineers